Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures during boosted conditions. At higher loads, the engine may be more knock-limited which may result in undesired late combustion phasing.
The engine knock may be addressed by retarding spark timing such as described in US20030131805 A1. In another example, systems with fixed long intake cams, such as in Atkinson engines and as shown in US20030131805 A1 as well, may be used for knock control.
The inventors herein have identified potential issues, including issues with the above approaches to addressing knock limits. For example, significant spark retard can reduce fuel economy and limit maximum torque. Further, fixed long intake cams may be optimized as a compromise between part and full load conditions.
The inventors herein have recognized the above issues and identified approaches to at least partly address the issues. In one example approach, a supercharged engine comprises at least one cylinder head with at least one cylinder, each cylinder having at least two inlet openings for the supply of charge air via an intake system and at least one outlet opening for the discharge of the exhaust gases via an exhaust-gas discharge system, at least one throttle flap which is arranged in the intake system and which serves for load control, and at least one exhaust-gas turbocharger, each exhaust-gas turbocharger comprising a turbine arranged in the exhaust-gas discharge system and a compressor arranged in the intake system. Further, at least two at least partially variable valve drives may be provided, the valve drives having at least two valves which may be movable between a valve closed position and a valve open position in order to open up and block the at least two inlet openings of the at least one cylinder, wherein a valve spring means for may preload the valves in the direction of the valve closed position. The at least partially variable valve drives may have at least two actuating devices for opening the valves counter to the preload force of the valve spring means, each actuating device comprising a cam which is arranged on a camshaft and which, as the camshaft rotates, may be brought into engagement with at least one cam follower element, whereby the associated valve is actuated, and the cams of the at least two actuating devices of the at least two at least partially variable valve drives may be rotatable relative to one another. Further, the at least two valves may be actuated based on a desired manifold pressure and a corrective factor based on a boost pressure.
In one example, the opening and closing of the inlet valves may be adjusted depending on the load and engine operating conditions. For example, closing time of the second inlet valve may be determined by determining a base closing time using a present engine speed and a present desired value for a pressure in an intake system, and corrected for turbocharging by determining an additive closing time based on boost pressure. In this way, a cam event may be elongated so that part of the air charge may be pushed back into the intake system in order to lower the actual compression ratio, which may result in increased efficiency, fuel economy, higher torque, and a combustion process that is less knock-limited. Further, a correction in the closing time of the inlet valve accounting for a knock regulator output, may be used to retard the closing of a second inlet valve further. In this way, the method may reduce or substantially eliminate the need for retarding ignition spark, thus further increasing efficiency. Overall, a turbocharged engine can be operated with less spark retard from maximum torque.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.